Semiconductor element

ABSTRACT

A semiconductor element comprises its main part constituted of a polycrystalline silicon semiconductor layer containing 0.01 to 1 atomic % of fluorine atoms.

This application is a continuation of application Ser. No. 203,613 filedJune 2, 1988, which is a continuation of 846,480, filed Mar. 31, 1986which is a continuation of 456,716, filed Jan. 10, 1983, all nowabandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a semiconductor element such as a field effectthin film transistor, and the like, more particularly to a semiconductorelement of which the main part is constituted of a polycrystallinesilicon thin film semiconductor layer which is of high quality inbehavior characteristics, reliability and stability.

2. Description of the Prior Art

Recently, for constitution of a scanning circuit portion of an imagereading device for use in image reading such as one-dimensionalphotosensor made in a continuous length of two-dimensional photosensorof an enlarged area, or for constitution of a driving circuit portion ofan image display device utilizing liquid crystal (abbreviated as LC),electrochromic material (abbreviated as EC) or electroluminescencematerial (abbreviated as EL), it has been proposed to form a fieldeffect thin film transistor by using as the base material a silicon thinfilm formed on a certain substrate, corresponding in size to theincreased area of such portions.

Such a silicon thin film is desired to be polycrystalline rather thanamorphous for realization of a large scale image reading device or imagedisplay device improved in having a higher speed and achieving a higherlevel of function. As one of the reasons, in spite of the requirementthat the effective carrier mobility μeff of a silicon thin film as basematerial for formation of a scanning circuit portion of such a highspeed, high function reading device or driving circuit portion of imagedisplay device should be large, that of the amorphous silicon thin filmobtained by the ordinary discharge decomposition method is at most 0.1cm² /V.sec, and, it also suffers from the drawback that the draincurrent is reduced as DC voltage is applied to the gate, whereby thereis a marked change with the lapse of time such as shifting of thethreshold voltage of the transistor, resulting in poor in stability.

In contrast, a polycrystalline silicon thin film has an effectivecarrier mobility μeff by far greater than an amorphous silicon thin filmas can be seen from the data practically measured. Theoretically, it isquite probable that there may be prepared a polycrystalline silicon thinfilm having a further increased value of mobility μeff than presentlyobtained.

However, under the present situation, a semiconductor element or asemiconductor device comprising as its main part a semiconductor layerof a polycrystalline silicon thin film prepared by various methods ofthe prior art does not exhibit sufficiently the desired characteristicsand reliability. There have been also made so many studies about thesemiconductor elements formed from amorphous silicon thin filmscontaining silicon and fluorine. However, thin film field effecttransistors in which amorphous silicons containing silicon and fluorineare used as semiconductor layers have small effective mobilities, andalso suffer from the drawback of a large change with lapse of time suchthat the drain current is reduced as a DC voltage is applied on the gatewith a shift of the threshold voltage of the transistor, thus being poorin stability.

SUMMARY OF THE INVENTION

The present inventors have made extensive studies in view of the abovepoints on the basis of an idea that the characteristics and reliabilityat the junction interface of most semiconductor elements havingjunctions of laminar structures (PN junction or MIS structure) willdetermine the performance and reliability of such elements, andconsequently found that in a polycrystalline silicon thin filmsemiconductor element, the content of fluorine atoms in the silicon thinfilm will determine the performance and reliability of the element.

A principal object of the present invention is to provide asemiconductor element having a polycrystalline silicon thin filmsemiconductor layer of high performance.

It is another object of the present invention is to provide a fieldeffect thin film transistor having high performance, high reliabilityand high stability, by use of a polycrystalline silicon thin filmsemiconductor formed on a substrate.

It is a further object of the present invention is to provide asemiconductor device with an enlarged area comprising field effect thinfilm transistors using excellent polycrystalline silicon thin filmsemiconductor layers as constituent elements.

According to the present invention, there is provided a semiconductorelement comprising its main part constituted of a polycrystallinesilicon thin film semiconductor layer containing 0.01 to 1 atomic % offluorine atoms (F).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic sectional view for illustration of thesemiconductor element of the present invention;

FIG. 2 shows a schematic view for illustration of the circuit formeasurement of the characteristics of the semiconductor element of thepresent invention;

FIGS. 3 and 5 each show a schematic sectional view for illustration ofan example of a device for preparation of a semiconductor film of thepresent invention.

FIG. 4 shows a schematic chart for illustration of the steps forpreparation of the semiconductor element of the present invention.

FIG. 6 shows a graph for illustration of one example of V_(d) -I_(D)characteristics of the semiconductor element of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The field effect type thin film transistor (TFT) as an example of thesemiconductor element of the present invention prepared using thepolycrystalline silicon thin film as the base material is known as atransistor constituted of a semiconductor layer, an electrode layer andan insulating layer. That is, a voltage is applied between the sourceelectrode and the drain electrode having ohmic contact adjacent to thesemiconductor layer, and the channel current passing between saidelectrodes is modulated by the bias voltage applied to the gateelectrode provided on the insulating layer.

FIG. 1 shows an example of a typical basic structure of such TFT. On asemiconductor layer 102 provided on an insulating substrate 101 thereare provided a source electrode 103 and a drain electrode 104 in contactwith said semiconductor layer, and an insulating layer 105 is providedso as to cover over these electrodes, and a gate electrode 106 providedon said insulating layer 105.

In a TFT having the structure as shown in FIG. 1 in this invention, thesemiconductor layer 102 is constituted of a polycrystalline silicon thinfilm having the characteristics as described above, and between thesemiconductor layer 102 and each of the two electrodes, namely thesource electrode 103 and the drain electrode 104, there are formed, forexample, a first n⁺ layer 107 and a second n⁺ layer 108 each beingconstituted of an amorphous silicon, thus forming ohmic contacttherebetween.

The insulating layer 105 may be constituted of a material such assilicon nitride layer, SiO₂ layer, Al₂ O₃ layer and the like which canbe formed according to the CVD (Chemical Vapour Deposition) method,LPCVD (Low Pressure Chemical Vapour Deposition) method or PCVD (PlasmaChemical Vapour Deposition) method.

As the reactive gas to be employed for preparation of thepolycrystalline silicon thin film constituting the semiconductor layer,there may be included substances comprising silicon as constituentatoms, as exemplified by SiF₄, Si₂ F₆, etc. which may also be diluted,if necessary, with gases such as H₂, Ar, He and the like.

The field effect type TFT as an example of the semiconductor element ofthe present invention, can be classified into a type in which a gateinsulating layer is provided on a gate electrode (lower gate type) and atype in which a gate electrode is provided on a gate insulating layer(upper gate type). On the other hand, it may also be classified into atype in which source and drain electrodes are provided at the interfacebetween the insulating layer and the semiconductor layer (Coplanar type)and a type in which source and drain electrodes are on the semiconductorplane confronting the interface between the insulating layer and thesemiconductor layer (Stagger type). Thus, there are four types ofcombinations in all, as is well known in the art. The structure as shownin FIG. 1 is called as upper gate Coplanar type field effect TFT. In thepresent invention, any one of these types of field effect type TFT maybe available as a matter of course.

In the present invention, various transistor chacteristics can beimproved by controlling the fluorine (F) atom content in thepolycrystalline silicon thin film constituting the semiconductor layeras the main part of the semiconductor element to a level of 0.01 atomic% or higher. The fluorine (F) atoms contained in the polycrystallinesilicon thin film seem to exist primarily at the grain boundary of thepolycrystalline silicon. From a number of experimental facts found bythe present inventors, it has been observed that at a level of 1 atomic% or less of fluorine (F) atom content, substantially no deterioration,especially change with lapse of time, of transistor characteristics willoccur, whereby the characteristics can be maintained stably. That is,for example, at a fluorine (F) atom level exceeding 1 atomic %, when thetransistor is continuously actuated as described above, it has beenobserved that the effective carrier mobility μeff is decreased, and theoutput drain current is decreased with lapse of time with change inthreshold voltage. In the present invention, the fluorine (F) atomcontent is generally 0.01 to 1 atomic %, but preferably 0.05 to 1 atomic%, most preferably 0.05 to 0.5 atomic %.

Measurement of the fluorine content in the polycrystalline silicon thinfilm in the semiconductor element of the present invention was conductedby means of a Fourier-transform infrared spectrometer (FTIR, produced byJAS Co., Model FTS15C/D), when the content was 0.1 atomic % or more.That is, as a support for FTIR, a mirror ground, silicon wafer substrateof a thickness of 500 μm and a size of 1 inch was washed thoroughlysuccessively with distilled water and with deionized water, defattedwith methyl ethyl ketone and washed with isopropanol, followed bydrying. The fluorine content in the film was determined in such a mannerthat the absorption intensity of absorption band at 850 cm⁻¹ which isthe stretching vibration mode of Si-F is integrated according to thehalf-width approximation method and the number of bound fluorine atomswas determined from the value obtained by the integration.

Measurement of a fluorine content less than 0.1 atomic % was conductedby means of a secondary ion mass spectrometer-SIMS-(Model IMS-3f,produced by Cameca Co.). A conventional method was followed in thisanalytical method. That is, for prevention of charge-up, gold was vapordeposited to a thickness of 200 Å on a thin film of the sample, andmeasurement was conducted under the conditions of an ion energy ofprimary ion beam of 8 KeV and a sample current of 5×10⁻¹⁰ Å, with a spotsize of 50 μm in diameter and an etching area of 250 μm×250 μm, todetermine the detection intensity ratio of F⁺ ion relative to Si⁺, fromwhich the fluorine content was calculated in terms of atomic %.

On the other hand, in order to verify the quantitative nature of thefluorine contents determined by the above FTIR method and the SIMSmethod, fluorine contents were measured by the lanthanum-alizarincomplexion absorptiometric method well known in chemical analysis,whereby coincidence of the FTIR method with the SIMS method wasconfirmed.

The change with lapse of time of the polycrystalline silicon thin filmtransistor for showing the effect of the present invention was measuredaccording to the method as described below.

A TFT having a structure as shown in FIG. 2 was prepared, and byapplying a gate voltage V_(G) =40 V to the gate 201 and a drain voltageV_(D) =40 V between the source 203 and the drain 202, the drain currentI_(D) flowing between the source 203 and the drain 202 was monitored byan electrometer (Keithley 610 C electrometer) to measure the change ofthe drain current with lapse of time. The percentage of the change withlapse of time was determined by dividing the change quantity of draincurrent after 500 hours continuous actuation by the initial draincurrent and multiplying the value obtained by 100 so as to berepresented in %.

The threshold voltage V_(TH) of TFT was defined, as conventionallyconducted in MOS FET (Metal Oxide Semiconductor Field EffectTransistor), as the point at which the line extrapolated from thestraight line portion in ##EQU1## curve crosses over the abscissa ofV_(D). The changes of V_(TH) before and after the change with lapse oftime were examined at the same time and the changed quantity wasrepresented in volt.

The F content in the polycrystalline silicon thin film semiconductorlayer can be restricted to the range as described above according tovarious methods. For example, it can be realized under the specificconditions according to the method in which a fluorinated silicon suchas SiF₄ and the like is deposited by the glow discharge decomposition(GD), the method in which sputtering is effected using a Si target in agas containing F₂ (SP), the method in which Si is subjected to vapordeposition by use of electron beam in a F₂ gas plasma atmospere (IP), aswell as the method in which a polycrystalline silicon thin film formedby CVD or LPCVD is subjected to the F₂ gas plasma treatment, and so on.To be specifically noted in the present invention, the polycrystallinesilicon thin film semiconductor layer formed by the GD method, the SPmethod and the IP method, even at a low temperature of 400° C. to 450°C., so long as the restriction of fluorine content is satisfied, cangive transistor characteristics comparable to those of a polycrystallinesilicon film conventionally known in the art as prepared by, forexample, CVD or LPCVD at a higher temperature (at 600° C. or higher)followed by H₂ plasma annealing, and also can give more stability andreliability than such conventionally known film, thus indicatingdirectly usefulness of the present invention.

Further, as the orientation at the (220) plane in the polycrystallinesilicon thin film is intensified while satisfying the fluorine contentin the thin film, the transistor characteristics, particularly theeffective carrie mobility is recognized to be further improved, wherebythe change with lapse of time on continuous actuation becomes little.

It is known in the art that polycrystalline silicon thin films can beobtained so as to have various crystallinity and orientationcharacteristics depending on the film preparation method and the filmpreparation conditions.

In the present invention, as the method for examination of theorientation characteristics, X-ray diffraction and electron beamdiffraction we e performed in combination.

X-ray diffraction intensities of polycrystalline silicon films preparedwere measured by X-ray diffractometer produced by Rigaku Denki (coppertube bulb, 35 KV, 10 mA) and comparison was made. The diffraction angle2 θ was varied from 20° to 65° and diffraction peaks at plane indices of(111) plane, (220) plane and (311) plane were detected to determinetheir diffraction intensities.

Electron beam diffraction intensities were measured by JEM-100 Vproduced by Nippon Denshi Co., and respective diffraction intensitieswere determined similarly. According to the ASTM card (No. 27-1977), incase of a polycrystalline silicon having no orientation at all, takingout only (220) from the planes in terms of (h, k, l) representation withgreater diffraction intensities of (111):(220):(311) =100:55:30, itsratio relative to the total diffraction intensities, namely diffractionintensity of (220)/(total diffraction intensity) is equal to(55/241)×100≈22.8 (%).

With the use of this value as the standard, an orientationcharacteristic (220) with a greater value than the above percentage,especially 30% or more, can give further improved transistorcharacteristics. At a value less than 30%, the change with lapse of timebecomes undesirably greater.

In the present invention, the maximum of the surface unevenness of thepolycrystalline silicon thin film constituting the main part of asemiconductor element can be made 800 Å or less to achieve the effect ofthe present invention more effectively. The polycrystalline silicon thinfilm which is formed with a surface unevenness of 800 Å or less at themaximum shows no marked difference adversely affecting the elementcharacteristics in crystallinity and orientation characteristic in thedirection of film thickness as the result of dense crystal growth takingplace from the substrate interface and can give good elementcharacteristics also in case of forming a transistor element.

It is desirable to make the maximum of surface unevenness of thepolycrystalline silicon thin film to preferably 800 Å or less, morepreferably 500 Å or less. Further, it is desirable to make the maximumof the surface unevenness of polycrystalline silicon thin filmconstituting a semiconductor layer forming the main part of asemiconductor element to 800 Å or less over the entire surface region ofthe semiconductor layer which constitutes actually the element.

In the present invention, measurement of the surface unevenness wasconducted by means of a field radiation type scanning electronmicroscope (Model JFSM-30, produced by Nippon Denshi Co.) and it wasdetermined from an image of 100,000 x magnification of the surfacecross-section of a polycrystalline silicon thin film with acceleratedelectrons under 25 KV incident on the surface obliquely.

As disclosed in the present invention, formation of a polycrystallinesilicon thin film suited for the object of the present invention may bepossible particularly by carrying out glow discharge decomposition (GDmethod) of a fluorinated silicon compound gas, sputtering of silicon inF₂ atmosphere (SP method) or ion plating method (IP method) at asubstrate surface temperature of 500° C. or lower (in the range of about400° to 500° C.). This fact is not only advantageous in uniform heatingof substrate or provision of a cheap substrate material of large area inpreparation of a driving circuit or a scanning circuit covering over alarge area for a large area device, but also important in that it cansatisfy the requirement that a light-transmissive glass plate isdesirably used as a substrate for transmissive-type display device or inapplication of an image device such as in case of a photoelectricconverting light-receiving element of the type in which light enters onthe substrate side.

Accordingly, since the present invention can be practiced even at lowertemperature zones as compared with the prior art techniques, low-meltingglasses in general, heat-resistant plastics, etc. may also be availablein addition to heat-resistant glasses such as high-melting glasses, hardglass, etc., heat-resistant ceramics, sapphire, spinel, silicon wafer,etc. conventionally used in the prior art.

As the glass substrate, there may be employed a regular glass having asoftening point of 630° C., an ordinary hard glass having a softeningpoint of 780° C., a ultra-hard glass having a softening point of 820° C.(JIS First grade ultra-hard glass), etc.

In the preparation method of this invention, an advantage is involved inthat films can be formed without impairing the substrate, because anysubstrate can be used at a substrate temperature which can be suppressedlower than the softening point of the substrate employed.

In Examples of the present invention, there was primarily employed asthe substrate glass Corning #7059 glass among the regular glasses (sodaglasses) having relatively low softening points, but it is of coursepossible to use a quartz glass having a softening point of 1,500° C. asthe substrate. However, from practical standpoint, the use of regularglasses is advantageous in preparation of thin film transistors, TFT atlow cost and over a large area.

The field effect thin film transistor (FE-TFT) as an example of thesemiconductor element prepared with the use of the polycrystallinesilicon thin film as the base material can be improved in transistorcharacteristics (effective carrier mobility, threshold voltage, ON/OFFratio, gm, etc.), without change of transistor characteristics withlapse of time on continuous running, and also improved in yield of theelement and further reduced in fluctuation of the characteristics.Therefore, it is suitable as an element constituting a scanning circuitor a driving circuit of a display device or image device utilizing LC(Liquid crystal), EL (Electroluminescence) or EC (Electrochromy).

In order to further illustrate the present invention, preparation ofpolycrystalline silicon thin films, preparation process of TFT and theresults of TFT behaviors are described in detail below by way ofExamples.

EXAMPLE 1

This example describes preparation of a TFT by formation of apolycrystalline silicon thin film on a substrate by means of the deviceas shown in FIG. 3. As the substrate 300. Corning #7059 glass was used.

First, after the substrate 300 was washed, it was lightly etched on itssurface with a mixture of (HF+HNO₃ +CH₃ COOH) and after drying mountedon a substrate heating holder 302 placed on the anode side in a bell-jarvacuum deposition chamber 301.

Then, the bell-jar 301 was evacuated by a diffusion pump 310 to abackground vacuum degree of 2×10⁻⁶ Torr or less. During this operation,if the vacuum degree is poor, a reactive gas does not effectively takepart in the film deposition, and moreover entrainment of oxygen atoms(O), nitrogen atoms (N) may also occur in the film to thereby changemarkedly the dark resistance of the film. Next, the substratetemperature Ts was raised and the substrate 300 was maintained at 500°C. (the substrate temperature was monitored by a thermocouple 303).

In this Example, as the reactive gase to be introduced, a gas mixture ofSiF₄ gas and H₂ gas was employed. The gas flow rate of SiF₄ wascontrolled to 20 SCCM through a mass flow controller 304, while that ofH₂ to 10 SCCM through a mass flow controller 305.

The pressure in the bell-jar 301 was set at 0.15 Torr by means of anabsolute pressure gage 313 by controlling the pressure control valve 311on the evacuation side of the bell-jar 301. After the pressure in thebell-jar 301 was stabilized, a high frequency electric field of 13.56MHz was applied from a power source 315 to the cathode electrode 314 tocommence glow discharging. The voltage was 0.5 KV, with the currentbeing 48 mA and RF discharging power 100 W. The film formed had athickness of 5000 Å and its uniformity degree was within ±10% relativeto the substrate dimension of 120×120 mm when employing a ring-shapedgas blowing outlet.

The film formed contained 0.2 atomic % of fluorine atoms.

The orientation characteristic of the above thin film was examined fromthe X-ray diffraction data so that it was 90% [=I(220)/I(total)×100].

As the next step, following the process as shown in FIG. 4, a TFT wasprepared using this film as the base material. As shown in the step A,after deposition of a polycrystalline silicon film 401 formed on theglass substrate 300 as described above, PH₃ gas diluted to 100 vol ppmwith hydrogen gas [abbreviated as PH₃ (100 ppm)/H₂ ] was permitted toflow into the bell-jar 301 at a molar ratio of 5×10⁻³ relative to SiH₄gas diluted to 10 vol % with H₂ [abbreviated as SiH₄ (10)/H₂ ] to adjustthe pressure in the bell-jar 301 to 0.12 Torr, whereupon glowdischarging was effected to form a n⁺ layer 402 doped with P to athickness of 500 Å (step B). Subsequently, as the step C, the n⁺ layer402 was removed by photoetching except for the regions of the sourceelectrode 403 and the drain electrode 404. Then, the above substrate wasmounted again onto the heating holder 302 on the anode side in thebell-jar 301 so as to form a gate insulating film thereon. Similarly asin formation of the polycrystalline silicon thin film, the bell-jar 301was evacuated, the substrate temperature Ts maintained at 250° C. andglow discharging was excited by introduction of NH₃ gas at 20 SCCM andSiH₄ (10)/H₂ gas at 5 SCCM, whereby SiNH film 405 was deposited to athickness of 2500 Å.

Next, contact holes 406-1 and 406-2 for the source electrode 403 and thedrain electrode 404 were made by the photoetching step, and thereafteran electrode film 407 was formed on the entire surface of the SiNH film405 by vapor deposition of Al, followed by working of the Al electrodefilm 407 by the photoetching step to form the lead-out electrode 408 forthe source electrode, the lead-out electrode 409 for the drain electrodeand the gate electrode 410. Thereafter, heat treatment was conducted at250° C. in H₂ atmosphere. The thin film transistor (channel lengthL=10μ, channel width W=500μ) formed according to the process under theconditions as described above was found to exhibit stable and goodcharacteristics.

FIG. 6 shows V_(D) -I_(D) curve as an exemplary characteristic of thethus prepared TFT. (In FIG. 6, V_(D) is the drain voltage, V_(G) is thegate voltage and I_(D) is the drain current.) At V_(G) =20 V, I_(D)=2.0×10⁻⁴ A, and at V_(G) =0 V, I_(D) =1×10⁻⁷ A, and the thresholdvoltage was 2.0 V. The effective mobility (μeff) as obtained from thestraight line portion of ##EQU2## curve as generally practiced inMOS-TFT devices was found to be 8.0 cm² /V.sec. Thus, a TFT having goodtransistor characteristics was obtained. For examination of stability ofthis TFT, DC voltage was continuously applied to the gate at V_(G) =40V, and measurement of change of I_(D) was continuously conducted over500 hours. As the result, there was substantially no change of I_(D),the change being within ±0.1%. And, there was also no change inthreshold voltage, ΔV_(TH), before and after the continuous measurement,thus indicating extremely good stability of the TFT. V_(D) -I_(D), V_(G)_(-I) _(D) characteristics were also measured after such a continuousmeasurement to find that they were not changed in comparison to thosebefore the continuous measurement, with μeff value also remaining thesame, 8.0 cm² /V.sec.

As shown in this Example, the TFT of which main part is constituted of apolycrystalline silicon thin film with a fluorine atom content of 0.2atomic % having the characteristics of 90% orientation and an averagecrystal grain size of 900 Å exhibits high performance.

EXAMPLE 2

According to the same procedure as in Example 1, a polycrystallinesilicon film was formed on a Vycor glass substrate under the conditionsof RF power (Po) of 150 W, SiF₄ flow rate of 20 SCCM, H₂ flow rate of 10SCCM and glow discharge pressure (Pr) of 0.12 Torr. The substratetemperature (Ts) was set from 350° C. to 700° C. at intervals of 50° C.until the film was formed to have a thickness of 0.5μ. Polycrystallinesilicon films having various fluorine atom contents and orientationcharacteristics were formed, and TFT's prepared according to the samemethod as in Example 1 by using the respective films were found to haveeffective mobility values (μeff) as shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Sample No.   1-1 1-2 1-3 1-4 1-5 1-6 1-7 1-8                                  __________________________________________________________________________    Ts      (°C.)                                                                       350 400 450 500 550 600 650 700                                  Fluorine                                                                              atomic                                                                             1.5 0.8 0.6 0.5 0.4 0.2 0.05                                                                              <0.01                                atom content                                                                          (%)                                                                   Orientation                                                                           (%)  28  58  80  85  90  95  95  90                                   degree                                                                        μeff (cm.sup.2 /                                                                        0.7 1.5 3.2 5.5 8.0 4.5 1.6 0.5                                          V · sec)                                                     I.sub.D change with                                                                   (%)  1.5 0.4 ≦0.1                                                                       ≦0.1                                                                       ≦0.1                                                                       ≦0.1                                                                       ≦0.1                                                                       2.0                                  lapse of time                                                                 __________________________________________________________________________

As seen from Table 1, the samples with fluorine atom contents in excessof 1 atomic % or less than 0.01 atomic % have an effective mobility ofless than 1 cm² /V.sec., and a sample with an orientation degree lessthan 30% has an effective mobility value less than 1 cm² /V.sec.Therefore, those samples are inferior in practical application.

Further, the sample at Ts=700° C. has a high orientation of 90%, but asmall content of fluorine atoms less than 0.01 atomic % makes theeffective carrier mobility (μeff) as small as 0.7 cm² /V.sec. so that itis also inferior in practical application.

From Table 1, it can be seen that the sample (1-1) with a fluorine atomcontent over 1 atomic % and the sample (1-8) with a fluorine atomcontent less than 0.01 atomic % are inferior in performances of TFT,namely μeff and change of I_(D) with lapse of time.

EXAMPLE 3

A Corning #7059 glass substrate prepared similarly as in Example 1 wasfixed in close contact with a substrate heating holder 302 on the anodeside in the bell-jar 301, and a polycrystalline silicon plate (notshown: purity 99.99%) was placed on the electrode plate of the lowercathode 314 so as to be opposed to the substrate 300. The bell-jar 301was evacuated by means of a diffusion pump to 2×10⁻⁶ Torr, and thesurface temperature of the substrate 300 was maintained at 450° C. byheating of the substrate heating holder 302.

Subsequently, SiF₄ gas was introduced into the bell-jar 301 whilecontrolling its flow rate through a mass flowmeter 304 to 0.25 SCCM, andfurther Ar gas introduced into the bell-jar 301 while controlling itsflow rate through a mass flowmeter 309 to 25 SCCM, followed by narrowingof the main valve 311 to set the inner pressure in the bell-jar at 0.005Torr.

After the inner pressure in the bell-jar was stabilized, a voltage of2.6 KV was applied to the lower cathode 314 by the high frequency powersource 315 of 13.56 MHz to excite glow discharging between thepolycrystalline silicon plate on the cathode 314 and the anode(substrate heating holder) 302 at a discharge power of 300 W, whereby afilm with a thickness of about 0.5μ was formed.

The thus formed polycrystalline silicon film was found to have afluorine atom content of 0.5 atomic and an orientation degree of 55%.

Subsequently, by utilizing a part of the above film, a TFT was preparedaccording to the same steps as in Example 1. This element exhibited aneffective mobility μeff of 2.2 cm² /V.sec., and when changes in I_(D),and Vth were measured under the conditions of V_(G) =V_(D) =40 V, after500 hours, I_(D) change was found to be 0.2%, while there was observedno change in Vth, thus exhibiting good stability.

COMPARATIVE EXAMPLE

For the purpose of comparison with the above sample, the followingsample was prepared and similar measurements were conducted.

A Corning #7059 glass substrate prepared similarly as in Example 1 wasfixed in close contact with a substrate heating holder 302 on the anodeside in the bell-jar 301, and a polycrystalline silicon plate (notshown: purity 99.99%) was placed on the electrode plate of the lowercathode 314 so as to be opposed to the substrate. The bell-jar 301 wasevacuated by means of a diffusion pump to 2×10⁻⁶ Torr, and the surfacetemperature of the substrate 300 was maintained at 350° C. by heating ofthe substrate heating holder 302. Subsequently, SiF₄ gas was introducedinto the bell-jar through a mass flowmeter 304 at 1 SCCM, and further Argas introduced into the bell-jar 301 through a mass flowmeter 309 at aflow rate of 25 SCCM, followed by narrowing of the main valve 311 to setthe pressure in the bell-jar at 0.005 Torr.

After the inner pressure in the bell-jar was stabilized, a voltage of2.6 KV was applied to the lower cathode 314 by the high frequency powersource 315 of 13.56 MHz to excite glow discharging between thepolycrystalline silicon plate on the cathode and the anode (substrateheating holder) 302. The RF discharge power (travelling wavepower-reflection wave power) was 300 W, whereby a film with a thicknessof about 0.5μ was formed.

The thus formed polycrystalline silicon film was found to have afluorine atom content of 2.5 atomic and an orientation degree of lessthan 30%.

Subsequently, a TFT was prepared according to the same steps (A-G) as inExample 1.

This element exhibited an effective mobility μeff of 0.2 cm² V.sec. andwhen changes in I_(D), and Vth were measured under the conditions ofV_(G) =V_(D) =40 V, after 500 hours, I_(D) change was found to bereduced by 12% and ΔVth was 3 V, thus exhibiting poor stability of TFT.

EXAMPLE 4

An example is to be described below, in which a thin film transistor wasformed with the use of a polycrystalline silicon thin film semiconductorlayer prepared by means of the ion-plating deposition device as shown inFIG. 5.

First, in a deposition chamber 503 which can be brought to reducedpressure, there was placed a nondoped polycrystalline silicon body to beevaporated 506 in a boat 507, and a Corning #7059 substrate was placedon the supports 511-1 and 511-2. After the deposition chamber 503 wasevacuated to a base pressure of about 1×10⁻⁷ Torr, a F₂ gas with purityof 99.999% was introduced through the gas inlet tube 505 into thedeposition chamber so that the partial pressure of the F₂ gas (P_(F2))might be 4×10⁻⁵ Torr. The gas inlet tube 505 employed had an innerdiameter of 2 mm and shaped at its tip in a loop having gas blowingopenings of 0.5 mm at intervals of 2 cm.

Then, high frequency of 13.56 MHz was applied to the high frequency coil510 (5 mm in diameter) to set the output at 100 W, whereby a highfrequency plasma atmosphere was formed at the inside portion of thecoil.

On the other hand, while rotating the supports 511-1 and 511-2, theheating device 512 had been brought to the state under actuation andheated to about 450° C. As the next step, the evaporating body 506 wasirradiated and heated by the electron gun 508, whereby permittingsilicon particles to fly. The electron gun had a power of about 0.5 KW,and a polycrystalline silicon thin film of 0.5μ was formed. With the useof this thin film, a thin film transistor was prepared according to thesame process a in the foregoing Examples. Table 2 shows the fluorineatom contents in the film and the effective mobility (μeff) of the thinfilm transistor prepared. At the same time, there are shown the resultsof the case when the film was prepared under a partial fluorine gaspressure P_(F2) of 2×10⁻⁴ Torr and the case when the film was formedwithout introduction of fluorine gas.

                  TABLE 2                                                         ______________________________________                                        Sample No.   4-1       4-2      4-3                                           ______________________________________                                        Partial pressure                                                                           0         4 × 10.sup.-5                                                                    2 × 10.sup.-4                           of fluorine gas,                                                              P.sub.F2 (Torr)                                                               Fluorine atom                                                                              0         0.8      2.0                                           content                                                                       (atomic %)                                                                    Orientation  28        45       45                                            degree                                                                        μeff (cm.sup.2 /V · sec)                                                       0.02      1.5      0.4                                           ______________________________________                                    

We claim:
 1. A semiconductor element comprising as its main constituentpart a polycrystalline silicon semiconductor layer containing 0.01 to 1atomic % of fluorine atoms, wherein said semiconductor layer has anX-ray diffraction pattern or an electron beam diffraction pattern, ofwhich the diffraction intensity at the (220) plane is 30% or more basedon the total diffraction intensity.
 2. A semiconductor element accordingto claim 1, wherein the substrate on which the polycrystalline siliconsemiconductor layer is formed is a glass.
 3. A field effect type thinfilm transistor comprising:a semiconductor layer constituted of apolycrystalline silicon thin film containing 0.01 to 1 atomic % offluorine atoms and formed with a surface unevenness of 800 Å or less atthe maximum; an electrical insulating layer having one side electricallyin contact with the semiconductor layer; a gate electrode provided onthe side of the insulating layer opposite to the side which is incontact with the semiconductor layer; and a source electrode and a drainelectrode provided on the semiconductor layer and electrically incontact therewith.
 4. A field effect type thin film transistor accordingto claim 3, wherein n⁺ semiconductor layers are provided respectivelybetween the semiconductor layer and each of the source electrode and thedrain electrode.
 5. A field effect type thin film transistor accordingto claim 4, wherein the n⁺ semiconductor layers each are of amorphoussilicon.
 6. A field effect type thin film transistor according to claim3, wherein the insulating layer is of a material selected from amongsilicon nitride, SiO₂ and Al₂ O₃.
 7. A field effect type thin filmtransistor according to claim 3, wherein the fluorine atoms contained inthe polycrystalline silicon thin film exist primarily at the grainboundaries of the polycrystalline silicon.